Non-invasive imaging and analysis is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process.
In the particular case of non-invasive in-vivo imaging and analysis of biological tissue structures or components, such as structures or components of the eye, it is desirable to measure the physical size of structures or components of the eye under various conditions, for example to monitor for the onset of glaucoma or for other ophthalmic related purposes. A non-invasive method with increased precision enables more accurate monitoring of conditions of the eye.
Various versions of optical coherence tomography (OCT), are currently used to image and analyze targets, such as the human eye, non-invasively. One version of OCT uses a super-luminescent diode (SLD) as the optical source, The SLD output beam has a broad bandwidth and short coherence length.
The technique involves splitting the output beam into a probe and reference beam. The probe beam is typically applied to the target to be analyzed. Light scattered back from the target is combined with the reference beam to form the measurement signal. Because of the short coherence length only light that is scattered from a depth within the target such that the total optical path lengths of the probe and reference are equal combine interferometrically.
Thus the interferometric signal provides a measurement of the scattering value at a particular depth within the target. By varying the length of the reference path length, a measurement of the scattering values at various depths can be measured and thus the scattering value as a function of depth can be measured, providing image or analytic information.
In conventional OCT systems depth scanning is achieved by modifying the relative optical path length of the reference path and the probe path. The relative path length is modified by such techniques as electro-mechanical based technologies, such as galvanometers or moving coil actuators, rapid scanning optical delay lines and rotating polygons. All of these techniques involve moving parts that have to move a substantial distance, which have limited scan speeds and present significant alignment and associated signal to noise ratio related problems.
Motion occurring within the duration of a scan can cause significant problems in correct signal detection. If motion occurs within a scan duration, motion related artifacts may be indistinguishable from real signal information in the detected signal, leading to an inaccurate measurement. Long physical scans, for larger signal differentiation or locating reference areas, increase the severity of motion artifacts. Problematic motion can also include variation of the orientation of a target surface (such as skin) where small variations can have significant effects on measured scattering intensities.
Non-moving part solutions, include acousto-optic scanning, can be high speed, however such solutions are costly, bulky and have significant thermal control and associated thermal signal to noise ratio related problems. Optical fiber based OCT systems also use piezo electric fiber stretchers. These, however, have polarization rotation related signal to noise ratio problems and also are physically bulky, are expensive, require relatively high voltage control systems and also have the motion related issues.
These aspects cause conventional OCT systems to have significant undesirable signal to noise characteristics and present problems in practical implementations with sufficient accuracy, compactness and robustness for commercially viable and accurate imaging and analysis devices. Therefore there is an unmet need for commercially viable, compact, robust, non-invasive imaging and analysis technology and device with sufficient accuracy, precision and repeatability to image or analyze targets and in particular to image and analyze biological structures.